Highly Efficient Lightweight Flexible Cu(In,Ga)Se2 Solar Cells with a Narrow Bandgap Fabricated on Polyimide Substrates: Impact of Ag Alloying, Cs and Na Doping, and Front Shallow Ga Grading on Cell Performance

Herein, lightweight, flexible Cu(In,Ga)Se 2 (CIGS) solar cells with a narrow bandgap of ≈1 eV are grown on polyimide substrates. The poor performance of the CIGS solar cells owing to a low growth temperature (≈400 °C) is considerably improved via Ag alloying, Na doping using alkali‐silicate‐glass thin layers (ASTLs) and the CsF postdeposition treatment (CsF‐PDT), and front shallow Ga grading (surface field; SF). Along with improved device process, a notably high conversion efficiency of 21.2%, low V OC deficit of 0.346 V, and high J SC of ≈40 mA cm −2 are achieved. Ag alloying and Na doping using ASTLs predominantly improve the CIGS bulk quality, while the CsF‐PDT and SF reduce carrier recombination at the CIGS/CdS interface and vicinity. Device simulations reveal that the SF increases the electrical field at the CIGS surface under the forward bias voltage close to V OC owing to electron injection from the CdS side, which increases the chemical potential. Thus, the SF effectively repulses holes and improves the interfacial property. Device simulations also reveal that a high CIGS absorber's quality is prerequisite to benefit from the SF. Thus, CIGS solar cells showing improved bulk quality due to optimum alkali doping and Ag alloying considerably benefit from the SF.